My understanding of it was that you had to run after 5pm as they were trying to replicate the race conditions where you cant just choose when you run. As per the title and description on the website which i've cut and pasted below, the races are run on Monday evenings. Thats what i am doing anyway!

A SERIES OF VIRTUAL 5K RACES ON MONDAY EVENINGS STARTING ON APRIL 20TH
Run your own race, on a route of your own choosing adhering to Government guidelines on social distancing and safety. Measure your 5K run on your own device. Run your race and submit your time on Eventrac between 5pm and Midnight (GMT) each Monday. You must pre-register for each race before 5pm on race day at www.eventrac.co.uk(link is external) Registration is free of charge.